Monocytes are being characterized for the expression levels of CD40, CD80, CD86, CD120b, and the gamma interferon receptor alpha chain. These studies have identified two patients with gamma interferon receptor alpha chain deficiency, and the molecular level of this defect is being actively examined. In addition, interferon gamma receptor beta chain deficiency has also been characterized in one patient at the molecular level. As a consequence of these studies, an intracellular flow cytometric method for evaluating STAT protein phosphorylation has been developed and validated. This approach continues to be applied to new patients and is also being used to evaluate patients after bone marrow transplantation.